STOCKHOLM – The Swedish Air Force and NATO jets on Tuesday tracked four Russian combat aircraft flying with their transponders turned off over the Baltic Sea, officials said.

The Russian planes — two long-range, nuclear-capable Tu-22M3 bombers and two Sukhoi Su-27 fighters — were flying in international airspace, according to Sweden's Armed Forces and alliance sources.

NATO said it scrambled Danish jets and Italian jets based in Lithuania early Tuesday to identify the Russian aircraft which it said were heading to the Russian Baltic exclave of Kaliningrad.

"The Russian military aircraft did not use their onboard transponder; they were not in contact with civilian Air Traffic Control and they were not on a pre-filed flight plan," a NATO military officer said on condition he not be identified by name in keeping with alliance practice.

Swedish Foreign Minister Margot Wallstrom said it was "unacceptable" for the Russian planes to be flying with shut-off transponders that are necessary for identifying aircraft on radar, calling it violation of international aviation rules.

"This has happened now on a number of occasions and in a very challenging way," Wallstrom told reporters in Stockholm. "We are tired of always having to protest against this kind of ... breach of rules."

NATO and Sweden, which is not a member of the alliance, have reported an increase in Russian air maneuvers over the Baltic Sea in recent years.

Tuesday's sighting comes as Finnish and Swedish military aircraft are preparing to train with U.S. fighters over the Baltic Sea, and American and NATO forces continue military exercises in the Baltic countries.

The Swedish military said the threat against Sweden had not grown but that the armed forces were watching the "increased activity" in the region.